For the optical examination of specimens, thin sections of the specimens are often produced and are then examined with a microscope, for example a light or electron microscope. In the case of biological specimens in particular, it is often advantageous to embed them in a plastic in order to immobilize them sufficiently. For production of a thin section of the specimen, the specimen must then be exposed at the desired cut surface. This can be accomplished, for example, manually using a razor blade. While this method requires little technical complexity, it is nevertheless often not possible to trim the specimen exactly enough. In particular, flat cut surfaces cannot be generated.
A trimming apparatus is therefore used for exact trimming. For this purpose, the specimen with its plastic sheathing is clamped in a specimen holder. The specimen holder is then secured in the trimming apparatus. A motor-driven milling cutter then removes material under observation through a stereomicroscope, until a milled surface of the specimen is exposed at the location to be examined.
After milling, the thin or ultra-thin sections of the specimen can be produced. For this, the specimen mount together with the specimen is secured in a cutting device, for example the microtome or ultramicrotome. The specimen to be cut must then be exactly positioned in the cutting device with respect to the knife in accurately positioned and rapid fashion. Care must be taken that neither the knife nor the specimen is damaged in this so-called “presetting” operation. It is correspondingly necessary to prevent inadvertent contact from occurring between knife and specimen.
It has therefore been common for some time, in the context of the presetting operation between knife and specimen, to observe through a stereomicroscope as the knife and specimen approach one another. This observation does not always, however, result in a reliable estimate of the spacing between the specimen and the knife edge. Technical presetting aids have therefore also been used for some time, for example a base-mounted illumination system; with this it is possible to illuminate a gap between the knife and specimen, and on the basis of the illuminated gap to allow a better evaluation of the spacing between specimen and knife. A light source below the knife allows the spacing between knife and specimen to appear as a bright gap. An approach to within a few micrometers is required so that in the context of a diamond knife, which permits a maximum cut thickness of approx. 0.3 μm, it is not necessary to wait too long for the first cut.
This accuracy is also necessary for adjustment of the knife swing and specimen swing, since if an angular error exists, the first cuts result only in fragments of the entire trimmed surface. This procedure during presetting requires a great deal of practice on the part of the user in question, and is moreover extremely time-consuming.
It is already known from DE 41 11 689 to provide a force sensor that is mounted on the specimen or knife. The cutting force is sensed with the aid of this force sensor, so that it is possible to ascertain when the first cut occurs. In order to carry out the method, once an initial coarse positioning of the knife with respect to the specimen has been performed, the spacing between specimen and knife is decreased at a high feed and cutting speed. As soon as the specimen touches the knife for the first time, the force sensor responds. From that time on, operation switches to a selectable (usually slower) cutting speed, and a specific desired cut thickness is maintained.
In order to minimize stress on the specimen and knife, however, the first cuts after presetting must also not be too thick. For many diamond knives, a cut thickness of 0.3 μm is considered the upper limit. This means that before cutting operation begins, alignment of the knife and specimen must be accomplished to an accuracy of a few micrometers without contact between the knife and specimen. The method proposed in DE 41 11 689 cannot guarantee this, however. The first contact between knife and specimen takes place at high speed, so that damage to the knife and specimen can thereby occur.
In order to improve the presetting operation and to automate presetting, with the intent of simultaneously avoiding uncontrolled contact between the knife and specimen, EP 544 181 has proposed a method and an apparatus for automatic presetting. For this, there is mounted on the knife holder a so-called limiting device which is provided in order automatically to ascertain the proximity of the specimen holder along with the specimen secured therein. At the same time, the drive device is intended to switch off in timely fashion so that damage to the knife blade and to the specimen is reliably prevented. To ensure this, a movable plate having a microswitch located behind it is provided on the knife holder. In the presetting operation, the specimen is moved toward this plate until the switching point of the microswitch is reached. A disadvantage of this method, however, is the fact that contact between the specimen and the plate is necessary. In addition, the microswitch does not have the requisite repeatability in the micrometer range.